1. Field of the Invention
The present invention relates to a digital distance relay which can determine in less than one cycle of the power line signal whether a circuit breaker needs to be tripped for purposes of fault protection. In particular, the present invention relates to a digital distance relay which uses sample N/2+1 of the power line signal taken after detection of a fault to cancel the DC offset in the first N/2 samples of the power line signal taken after detection of the fault, where N is the number of samples taken per cycle of the power line signal, and then uses the DC compensated N/2 samples to calculate the N/2 samples expected in the latter half of the current cycle of the power line signal. The N/2 actual samples and N/2 calculated samples are then provided to a Full Cycle Fourier Algorithm for a determination of whether the detected fault is in the protection zone of the circuit breaker and hence whether the circuit breaker needs to be tripped to protect the AC electric power transmission line system from the fault.
2. Description of the Prior Art
As well known by those skilled in the art, AC electric power generating systems are typically interconnected in a complex power grid by high voltage alternating current (AC) three-phase electric power transmission lines. Faults occur on the transmission lines when a conductor wire breaks and falls to the ground or when the conductors short-circuit together. In the event of such faults, the power grid is provided with circuit breakers for disconnecting the faulted section of the transmission line in order to protect the power line equipment. When properly controlled by a digital distance relay or a distance-relaying computer, the faulted section, and only that section, is promptly disconnected from the power grid to avoid unnecessary interruptions of service to the electric power consumers and to prevent a power blackout from extending over an unnecessarily large geographic region.
Digital distance relays protect AC electric power transmission systems from faults by providing trip signals to the circuit breakers in the protection zones of the circuit breakers. In general, it is desired that the digital distance relay provide a trip signal to the circuit breaker as early as possible in the first AC cycle after the inception of the fault without sacrificing the accuracy of determining the protection zone in which the fault occurred so that damage to the power transmission equipment can be avoided. However, the determination of whether the detected fault is in the protection zone of the associated circuit breaker typically requires a complex determination of impedance variations on the power transmission lines, which, in turn, requires at least one cycle of sample data of the AC electric power cycle before a reliable distance determination can be made.
Digital distance relays typically sample the line currents and voltages from the three phases of a transmission line and detect a fault when there is an unusual departure of the sample values from the expected values. Once a fault is detected, post-fault voltage and current samples are taken for use in calculating the distance from the relay to the fault. Unfortunately, these samples are typically corrupted by noise and transient behavior in the first post-fault cycle of the power line signal, thereby limiting the speed with which the steady-state sinusoidal waveform of the power line signal can be identified for use in making accurate impedance calculations for determining where along the transmission line the fault occurred. The sooner accurate impedance calculations can be made, the sooner the faulted line section can be accurately identified.
FIG. 1 illustrates a generic prior art digital distance relaying system for detecting a fault in a three-phase electric power transmission line and appropriately tripping a circuit breaker in the event the fault is determined to be in the protection zone of that circuit breaker. In particular, FIG. 1 illustrates a portion of a three-phase 60 Hz electric power grid comprising a three-phase transmission line having conductors A, B and C which are fault protected by a digital distance relay. Electric power network 100 includes the electrical generation and transmission equipment to the left of transformer 102 and is represented as an equivalent circuit including an equivalent generator 100. Network 100 delivers electric power to transformer 102 and to AC electric power lines A, B and C as illustrated. When a fault F occurs at some point along transmission lines A, B and/or C, circuit breaker 104 is promptly tripped to disconnect transmission lines A, B and C from network 100. For this purpose, current transformers 106 and potential (voltage) transformers 108 are provided for respectively monitoring the current and voltage on each of the power transmission lines A, B and C.
The currents of the respective power lines A, B and C (I.sub.A, I.sub.B, and I.sub.C) are provided to low pass analog filter 110 to prevent aliasing. Similarly, the power line to ground voltages V.sub.AG, V.sub.BG, and V.sub.CG of the respective power lines A, B, and C are provided to low pass analog filter 112 to prevent aliasing. The filtered signals from low pass analog filters 110 and 112 are then converted to digital form by respective A/D converters 114 and 116 and then provided to computer 118 where the digital values are stored in data buffers for processing. Of course, a single A/D converter with an appropriately sized single data buffer may be used for this purpose.
Computer 118 processes the received digital data to determine from the received data whether the fault occurred in the protection zone of the circuit breaker 104 and, if so, to provide a trip signal to circuit breaker 104. Typically, computer 118 comprises a suitable minicomputer or microprocessor architecture having sufficient operating speed and memory capacity to calculate the impedance of the transmission line from the point of the relay to the fault location as required for the protective relay function in accordance with known impedance distance relaying techniques. Additional memory 120 may be provided for storing programs and/or additional data for use by computer 118.
Different techniques for detecting the presence of a fault in the protection zone of the circuit breaker 104 have been implemented in the prior art. As noted above, a typical prior art technique includes detecting faults based upon changes in voltage and/or current values on the three-phase power lines. Once a fault has been detected, a suitable algorithm is implemented to determine impedance variations on the power lines to thereby determine the distance from the relay to the fault. Many algorithms have been implemented for this purpose. For example, a Fourier transform technique is widely used in which a "window" of data sampled after the fault is processed to determine whether the fault is in the protection zone of the associated circuit breaker. The sampling of the steady-state post-fault voltage and current sinusoids typically begins as soon as possible after the fault. Then, once a "window" of samples sufficient to represent a full cycle of the power line signal is obtained, the data is processed in accordance with a well-known Fourier transform technique to provide an estimate of the distance from the circuit breaker to the fault. Although quite accurate, this technique requires at least a full cycle of post-fault samples to generate a trip signal. It is desired to shorten this time period to minimize fault current damage.
A protective relay system of the type illustrated in FIG. 1 is described by Girgis et al. in U.S. Pat. No. 4,455,612. Girgis et al. implement a recursive estimation technique on computer 118 which allegedly provides an accurate estimation of the post-fault voltage and current phasors so that a trip signal can be provided during the first post-fault electrical cycle, even though the line voltage and currents are typically affected by noise transients. Since the steady-state information is desired during the first cycle after the fault (before the noise transient has decayed), the steady-state waveforms must be estimated. Girgis et al. do this with a digital distance relay which responds to each post-fault sample as it arrives to recursively electronically estimate a waveform including the steady-state voltage and current sinusoids before the next voltage or current sample arrives. For this purpose, a state variable approach having no "window" length requirements is used. The recursive nature of the method permits an updated estimate to be made utilizing the latest sample of data and the previous estimate, thereby utilizing all of the information in all previous post-fault samples without repeating the earlier calculations. Such a technique purports to provide relaying within the first half of the post-fault electrical cycle for most Zone 1 faults. However, several microprocessors are necessary for each relay in order to implement the recursive algorithm described by Girgis et al. and, accordingly, such a technique is very expensive to implement in practice.
Another protective relay system of the type illustrated in FIG. 1 is described by Sackin et al. in U.S. Pat. No. 4,321,681. In that system, the outputs of two separate algorithms operated on computer 118 are logically related to provide the fastest possible trip depending upon the noise transient conditions of the post-fault voltage and current waveforms. The first algorithm is the aforementioned Full Cycle Fourier algorithm which provides accurate results notwithstanding severe waveform distortion but requires a full cycle or "window" of sample data. To increase tripping speed in the event that the fault is close to the relay and/or there are few noise transients after the fault, Sackin et al. also implement a mathematical algorithm which requires as few as three samples to predict the peak of the current sinusoid by using the first and second derivatives of the sampled values. Thus, the algorithm has an aperture or data window of three samples. However, in order to correctly predict fault current magnitude using this algorithm, the three consecutive samples must follow the fault inception point and make a trip/no-trip decision based upon this data. Unfortunately, the fault waveforms immediately after the fault inception point are seldom pure sinusolds since they are affected by the aforementioned noise transients. In practice, the fault waveforms are usually distorted to include a DC offset transient which may have a magnitude as large as the fault current peak. Moreover, line reactance prevents an instantaneous change in current from load to fault value, creating a decaying exponential DC transient as the system changes from a pre-fault steady-state condition to a post-fault steady-state condition. The voltage and current waveforms may also include other types of distortions such as harmonics, transients, and other high frequency noise such as that caused by non-linear elements, surge reflections, current transformer saturation, and the like. As a result, the mathematical algorithm which makes its decision after three data samples can seldom be used to produce accurate results. Accordingly, even though the protective relay apparatus disclosed by Sackin et al. uses the Fourier algorithm and three sample algorithm in a complementary manner, the disclosed apparatus usually must process a full cycle of data before a reliable trip signal can be provided.
Other prior art systems have implemented some sort of post-algorithm averaging or filtering in order to stabilize the output signals to provide data from which intelligent trip/no-trip decisions may be made. However, this typically requires many more samples to be taken before a post-fault steady-state calculation may be reached, thereby extending the data window and thus the time following fault inception before an accurate trip/no-trip decision may be made. Indeed, prior art Fourier and Walsh type algorithms have been developed in which the digital data is digitally filtered in the algorithm itself so as to provide band pass characteristics central about a power frequency and to provide a steady-state, accurate impedance calculation using one full cycle of samples following fault inception. However, as noted above, these algorithms require a data window equal to one full power frequency cycle. It is desired to shorten this time period to minimize the possibility of damage to the power transmission circuitry.
FIG. 2 illustrates yet another digital distance relay algorithm which may be implemented on computer 118. In particular, FIG. 2 illustrates a prior art MDAR digital distance relay currently used by the assignee of the present invention. The illustrated algorithm has a typical trip time of approximately 1.5 cycles and a maximum trip time of two cycles for Zone 1 and pilot operations. One cycle is required to implement the full cycle Fourier algorithm, while the additional time is required for digital filtering and the tripping relay delay. However, for a very close-in fault, the MDAR's trip time may be reduced to less than one cycle by implementing a "Restrict" mode in which a half cycle of data sampled before the fault and a half cycle of data sampled after the fault are used to fill the data window of the Full Cycle Fourier Algorithm. The Fourier algorithm then computes the one cycle sum-of-squares and Fourier sums. However, the transformer in-rush DC offset and the solid-state (analog) circuit's offset are not filtered out, and as a result, a severe overreach cannot be avoided.
The algorithm illustrated in FIG. 2 (to be described in more detail below) provides two paths or modes for tripping upon detection of Zone 1 faults: a restrict mode for rapid tripping for very close-in faults and the standard mode for implementing the Full Cycle Fourier Algorithm. As just noted, the restrict mode is used for very close-in faults, while the standard mode is used for a more accurate calculation of the distance to all other faults. In the standard mode, the first and second samples after the fault condition is detected are discarded in order to avoid the error due to the low-pass anti-aliasing filter. Thus, if eight samples are taken during each cycle of the power signal waveform, samples #3 to #10 after the fault are used for the first fault calculation. The Full Cycle Fourier Algorithm extracts the fundamental power line frequency phasor components from these eight current and voltage samples. The eight samples per cycle are required to define a given vector, and preferably, the Full Cycle Fourier Algorithm filters out the DC component automatically. As noted above, this technique is quite accurate, but unfortunately, it has a trip time of approximately 23 milliseconds for a 60 Hz transmission system (2 samples for the filter, 8 samples for the calculation, and 2 milliseconds for tripping relay delay) and is the fastest speed that the relay can achieve in the system illustrated in FIG. 2. It is desired that an accurate trip/no-trip determination be made in less than 1 cycle of the power line signal in order to minimize damage to the power transmission components. In particular, a faster and more reliable technique for generating a "window" of samples for use by the Fourier algorithm is desired. The present invention has been designed for this purpose.